The potent adenosine deaminase inhibitor, 2'-deoxycoformycin, inhibits the enzymic inactivation and potentiates the cytotoxic activity of a variety of adenosine analogs in the P388 murine leukemia cell culture system. Tubercidin (7-deaza-adenosine) is not a substrate for adenosine deaminase and its cytotoxicity in vitro was not potentiated by 2'-deoxycoformycin. Both 2'-deoxycoformycin and erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA) potentiate the antitumor activity of xylosyladenine against leukemia P388 and produce long term survivors. In addition 2'-deoxycoformycin was shown to have immunosuppressive activity in mice using an allograft tumor test system. BIBLIOGRAPHIC REFERENCES: Adamson, R.H., Zaharevitz, D.W. and Johns, D.G.: Enhancement of the Biological Activity of Adenosine Analogs by the Adenosine Deaminase Inhibitor 2'-Deoxycoformycin. Pharmacology. 15: 84-89, 1977. Chassin, M.M., Chirigos, M.A., Johns, D.G. and Adamson, R.H.: Adenosine Deaminase Inhibition for Immunosuppression. New England J. of Medicine. 296: 1232, 1977.